A circle with circumference $4\pi$ has an arc with a $90^\circ$ central angle. What is the length of the arc? ${4\pi}$ ${90^\circ}$ $\color{#DF0030}{1\pi}$
The ratio between the arc's central angle $\theta$ and $360^\circ$ is equal to the ratio between the arc length $s$ and the circle's circumference $c$ $\dfrac{\theta}{360^\circ} = \dfrac{s}{c}$ $\dfrac{90^\circ}{360^\circ} = \dfrac{s}{4\pi}$ $\dfrac{1}{4} = \dfrac{s}{4\pi}$ $\dfrac{1}{4} \times 4\pi = s$ $1\pi = s$